1. Field of the Invention
This invention relates to a method of coagulating a fluoro-elastomer in the form of a dispersion in an aqueous medium whereby in a simple one-step process a fluoro elastomer is formed which uniformly contains a cross-linking system. More particularly, this invention is directed to a process for coagulating a fluoro-elastomer wherein the final cross-linked product has superior properties. This invention is particularly directed to an economic means for the coagulation of a fluoro-elastomer from an aqueous acidic medium.
2. Discussion of the Prior Art
Elastomeric copolymers of vinylidene fluoride (VF.sub.2), hexafluoropropylene (HFP) and/or pentafluoropropylene (PFP) and in some cases other fluorinated olefins such as vinyl fluoride (VF) or tetrafluoroethylene (TFE) are used mainly in a cross-linked form for the manufacture of seals, hoses, coatings, and the like which are resistant to heat, chemicals and oils. These copolymers are obtained as a rule by free radical copolymerization in an aqueous phase. Hence, the result of the polymerization is a dispersion or latex of polymer. The fluoro-elastomers are then precipitated from the dispersion whereby a coagulate is formed. The coagulate is then mechanically separated from the aqueous phase, washed and dried and prepared. It is often prepared in the form of a slab or block. Finally, there is added thereto, usually by a fabricator, a vulcanizing additive and a vulcanizing agent which assists in the cross-linking.
Precipitating agents heretofore employed are usually inorganic coagulants such as hydrochloric acid or a potassium alum or a magnesium chloride. One difficulty with such a system lies in the necessity of separating the coagulant virtually completely from the isolated polymer before the latter is used. If this is not done delays are encountered in the vulcanization of the elastomer and, under certain circumstances, the properties of the vulcanized end product are impaired.
It has previously been proposed to employ as a coagulant small amounts of water-soluble aliphatic polyamine. Such has been proposed in German Auslegeschrift No. 2,008,192. The amount of water soluble aliphatic polyamine unfortunately had to measured quite precisely. On the one hand it was required to be small enough so that residues remaining in the polymer would have no adverse effect on the properties of the polymer in the subsequent drying and vulcanization. On the other hand, it had to be present in sufficient quantity to produce as complete a coagulation as possible. While on a laboratory scale it may have been possible, in operations on a large commercial or technical scale it proved to be quite difficult to maintain the amount of polyamine within the very narrowly defined ranges for suitable operation. Even very slight excesses or shortages resulted in end products having undesirable properties.
Another disadvantage in the use of the known coagulants resided in the fact that a fine distribution of the required vulcanization system consisting of the vulcanizer and the vulcanization accelerator after the coagulation or after the slabbing presented greater difficulties. For instance, a uniform distribution of the vulcanizing system in the fluoro-elastomer was essential to the preparation of a uniform vulcanizate having optimum reproducible properties.
It therefore became desirable to provide a simple means whereby the fluoro-elastomer could be readily coagulated or precipitated from the aqueous medium so that the same would be in a condition ready for a vulcanization step. More particularly, it became desirable to provide a new coagulating agent which would readily coagulate the fluoro-elastomer latex whereby the same would be in a condition for vulcanization without subsequent addition of vulcanization accelerators. More especially, it became desirable to provide a means for coagulating the fluoro-elastomer latex whereby the final vulcanized product had superior properties.